In the production of malt fermented beverages such as beer, happoshu (low-malt beer), whisky, etc., the major three sugars contained in a wort prepared by mashing a malt, etc. are glucose, maltose and maltotriose. The ratio of these malt-derived sugars can be somewhat varied depending on the mashing process and may be approximately 1:5:1, since the ratio does not change significantly when enzyme preparations, glycosylated starch, etc. are not added. Among them, glucose is a monosaccharide and preferentially assimilated as a sugar most favored by yeast.
Yeast has numerous genes suppressed in the presence of glucose during the transcription process. This suppressing control mechanism is called glucose repression. Several transporters required for uptake of maltose or maltotriose into yeast all undergo this repression. It is known that some of these gene products which undergo such glucose repression are inactivated in the presence of glucose even after translation. α-Glucoside transporters are also within this type and known to be rapidly degraded in the presence of glucose. The first step of assimilation of maltose or maltotriose is its uptake into yeast cells by these transporters and when transporters are degraded, assimilation of these sugars is discontinued. This is the reason why the expression of transporter is called a rate-determining step for assimilation.    Non-Patent Literature 1: Brondijk, T. H., van der Rest, M. E., Pluim, D., de Vries, Y. de., Stingl, K., Poolman, B., and Konings, W. N. (1998) J. Biol. Chem., 273 (25), 15352-15357    Non-Patent Literature 2: Medintz, I., Wang, X., Hradek, T., and Michels, C. A. (2000) Biochemistry, 39 (15), 4518-4526    Non-Patent Literature 3: Gadura, N., and Michels, C. A. (2006) Curr. Genet., 50 (2), 101-114